1. FIELD OF THE INVENTION
The present invention relates to a starter for an internal combustion engine, and more particularly to a system for preventing damage to the starter under certain operating conditions.
2. DESCRIPTION OF THE PRIOR ART
A well known type electric starter, for cranking an internal combustion engine, utilizes a solenoid that has a plunger which is connected to a pinion by a shift lever. A typical prior art starter of this type is shown and described in U.S. Pat. No. 4,674,344, "Engine Starter". When the coils of the solenoid are energized the plunger is shifted to cause the pinion to mesh with the ring gear of the engine. Movement of the plunger also causes a moveable contact to engage stationary contacts of a solenoid switch to energize the electric cranking motor and thereby rotatably drive the pinion.
In some starters the starter solenoid incorporates a two coil design to engage the starter pinion gear into the engine fly wheel ring gear for engaging the starter motor. In such an arrangement the solenoid coils may typically be referred to as the "attracting" and "holding" coils. The attracting and holding coils are simultaneously energized, with the attracting coil being sized and designed to exert a high magnetic force on the starter solenoid contactor arm to pull the pinion gear into engagement with the ring gear. In one known design the attracting coil requires a high current of approximately 35 amps and is only designed to be energized for a brief period of time (typically less than one second) to engage the pinion and ring gear. Once engagement of the pinion gear is achieved, the attracting coil is de-energized, and a separate holding coil remains energized to hold the pinion gear engaged with the ring gear. The lower output force holding coil exerts only enough force to hold the gears engaged and is designed to be engaged for longer periods of time. Typically, the holding coil draws approximately six to seven amps of current.
When the plunger is successfully shifted to cause the pinion to mesh with the ring gear, a fixed contact carried by the plunger closes a set of fixed contacts to thereby energize the cranking motor and the starter drive turns over the internal combustion engine. Successful engagement of the fixed contacts by the plunger contact also serves to deactivate the attracting coil.
In certain circumstances, usually when insufficient battery voltage is applied to the starter solenoid, the pinion gear and the ring gear do not fully engage. When this occurs, the moveable contact carried by the plunger does not engage the fixed contacts and the attracting coil remains energized as long as the starter solenoid is energized. As a result, the attracting coil and the starter may fail due to overheating of the windings caused by the high current flow therethrough.